Electrooptic light modulators utilizing bulk inorganic crystals are well-known and widely utilized. Waveguide electrooptic light modulators are a more recent development, and are described in literature such as Applied Physics Letters, 21, No. 7, 325 (1972); 22, No. 10, 540 (1973); and U.S. Pat. Nos. 3,586,872; 3,619,795; 3,624,406; 3,806,223; 3,810,688; 3,874,782; 3,923,374; 3,947,087; 3,990,775; and references cited therein.
One of the principal advantages of an optical waveguide configuration as contrasted to bulk crystals is that much lower electrical potentials may be used with the optical waveguide configuration, and much lower capacitive values and faster modulation rates also may be realized. Both of these operative characteristics are necessary to achieve high speed operation of such electrooptic modulators.
A thin film waveguide electrooptic modulator can operate employing one of several modulating mechanisms, e.g., phase retardation, Mach-Zehnder interferometry, directional coupling, or rotation of the optical polarization. Illustrative of waveguide designs are linear waveguide channels, such as those with a directional coupler configuration or a cross-bar configuration.
The guided-wave Mach-Zehnder interferometric modulator is a well-known optical device which has been described in literature such as "Multigigahertz-Lumped-Element Electrooptic Modulator," by R. A. Becker, IEEE Journal of Quantum Electronics, Vol. QE-21, No. 8, Aug. 1985, pp. 1144-1146; and "Guided-Wave Devices for Optical Communication," by R. C. Alferness, IEEE Journal of Quantum Electronics, Vol. QE-17, No. 6, June 1981, pp. 946-959.
The interferometric modulator consists of a single input waveguide, an input branching region for splitting the input light power into two substantially equal branch waveguides, an output branching region for recombining the propagating light power in the two branch waveguides, and an output waveguide. By effecting a phase shift in one branch waveguide relative to the other, the combined output light power is between zero and the input power level, depending upon the magnitude of the phase shift. Such phase shifts are effected by means of electrodes disposed on the substrate of the optical waveguide proximate to one or both of the branch waveguides. When a voltage is applied, the electrooptic effect changes the refractive index of the proximate branch waveguide changing the optical path length, thereby effecting a phase change in the branch. By keeping the branch waveguides sufficiently apart to prevent optical coupling between the branches which would degrade performance, voltage variations are linearly transformed into the phase changes and thus into amplitude variations in the light output power level. By modulating the electrode voltage with an analog or digital information signal, the output light power is similarly modulated and can be coupled onto a fiber waveguide for transmission.
There are other factors of critical concern in the design and fabrication of optical waveguides. The polarization properties of integrated optical switches and modulators are of great importance in determining the utility of these devices of an optical data transfer system employing fiber transmission lines. In particular, these devices must perform efficient and complete switching of light, without regard to its state of polarization. This requirement arises because linearly polarized light coupled into single-mode circular fibers suffers a rapid conversion to other polarization states. Light coupled from a fiber therefore usually possesses an unknown elliptical polarization, and both transverse electric (TE) and transverse magnetic (TM) modes will be excited in the integrated optical circuit. Any optical modulator must act in identical fashion upon each of the constituent polarizations in order to achieve an acceptable low level of interchannel crosstalk.
Polarization-independent optical switches and modulators are described in U.S. Pat. Nos. 4,243,295; 4,291,939; 4,514,046; 4,674,829; 4,756,588; and references cited therein. The known polarization-independent waveguide devices all are constructed with inorganic waveguide channels such as crystalline LiNbO.sub.3, LiTaO.sub.3, GaAs or CdSe.
Another important factor in the design and fabrication of optical waveguide devices is optical loss. In an optical data transfer system employing fiber transmission lines low optical loss properties of integrated optical switches and modulators are a critical requirement in conserving optical signal intensity and thereby enhancing the utility of the system. It is essential that these devices receive the optical signal transmitted by a fiber with a minimum of optical signal loss, and that the optical signal loss by scattering in the waveguide is minimized.
For these devices to receive the optical signal from the fiber transmission or to transfer the modulated optical signal to the fiber with a minimum of optical loss, the spatial mode profile of the optical signal in the waveguide of these devices must overlap as closely as possible with the mode profile in the optical fiber. This necessitates that the lateral dimensions of waveguides closely approximate the diameter of core region of the optical fiber.
As a further aspect of efficient electrooptic modulation, the waveguide of an electrooptic modulator must support only a single optical mode. This implies that the refractive index of cladding layers must be controlled precisely at a slightly lower value than that of the waveguide medium.
A detailed treatise on efficient coupling of optical signals between waveguiding media is provided in literature such as "Guided-Wave Optoelectronics", T. Tamir (ed.), Springer-Verlag, Chapter 3, pp 87-144 (1988).
For a low voltage operating electrooptic modulator, highly responsive nonlinear optical media are required. LiNbO.sub.3 has been an important inorganic species for waveguide electrooptic modulator construction. However, there are certain inherent disadvantages in the use of LiNbO.sub.3 or other inorganic crystal in an electrooptic modulator, such as the limitation of the input optical power due to the inherent photorefractive effect, and the high fabrication cost for a LiNbO.sub.3 high quality crystal.
It is known that organic and polymeric materials with large delocalized .pi.-electron systems can exhibit nonlinear optical response, which in many cases is a much larger response than by inorganic substrates.
In addition, the properties of organic and polymeric materials can be varied to optimize other desirable properties, such as mechanical and thermoxidative stability and high laser damage threshold, with preservation of the electronic interactions responsible for nonlinear optical effects.
Of particular importance for conjugated organic systems is the fact that the origin of the nonlinear effects is the polarization of the .pi.-electron cloud as opposed to displacement or rearrangement of nuclear coordinates found in inorganic materials.
Nonlinear optical properties of organic and polymeric materials was the subject of a symposium sponsored by the ACS division of Polymer Chemistry at the 18th meeting of the American Chemical Society, September 1982. Papers presented at the meeting are published in ACS Symposium Series 233, American Chemical Society, Washington D.C. 1983.
Organic nonlinear optical medium in the form of transparent thin substrates are described in U.S. Pat. Nos. 4,536,450; 4,605,869; 4,607,095; 4,615,962; and 4,624,872.
The above recited publications are incorporated herein by reference.
There is continuing research effort to develop new nonlinear optical organic media and electrooptic devices adapted for laser modulation, information control in optical circuitry, and the like. The potential utility of organic materials with large second order and third order nonlinearities for very high frequency application contrasts with the bandwidth limitations of conventional inorganic electrooptic materials.
Accordingly, it is an object of this invention to provide a novel waveguide medium for a low voltage operating electrooptic light modulator.
It is another object of this invention to provide an electrooptical light modulator which contains an organic nonlinear optical waveguide channel configuration, and which exhibits low optical loss properties.
It is a further object of this invention to provide a polarization-insensitive polymeric thin film waveguide electrooptic light amplitude modulator.
Other objects and advantages of the present invention shall become apparent from the accompanying description and drawings.